Oscilloscope error

1. The problem statement, all variables and given/known data
"An instrument (oscilloscope) is prepared to measure a sinusoidal periodic wave, compute the measurement error if the same instrument is used to measure the wave indicated in figure 1

(The wave is much more like a sawtooth wave, sorry for bad drawing)

2. Relevant equations
((Measured-True)/(Measured)) × 100

3. The attempt at a solution

Actually, I don't how to start, should I assume that sinusoidal wave is 4V peak (from 6V to 10V above, and 6V to 2V below) and substracting the values?